Imaging arrays comprised of multiple sensor pixels 100 are well known in the imaging art. Sensor pixels 100 typically include a switching element 110 such as a thin-film transistor (TFT) and a photoelectric conversion element 120 such as a photodiode. FIG. 1A shows a schematic equivalent pixel circuit for sensor pixels 100. The photoelectric conversion element 120 is sensitive to incident radiation and can generate a number of charge carriers where the number depends on the radiation dosage. In imaging arrays composed of a plurality of pixels, the photogenerated carriers are usually temporarily stored either across the internal capacitance of the sensor or in an on-pixel storage capacitor prior to readout. A signal of interest is usually represented by a potential change in the floating node 160 of the pixel. A task of the switching element 110 is to maintain the signal within the pixel and subsequently release the photogenerated carriers for readout. Aside from the two device elements 110, 120, the signal lines are also crucial to the functionality of the sensor pixel and the imaging array. The switching element 110 is controlled by scan line 130 that dictates the time and duration of the signal charge release process. The data line 140 provides a path for the charge carriers to the readout electronics. The bias line 150 provides the appropriate bias voltages for the photoelectric conversion elements 120.
A plurality of sensor pixels can be tiled in a matrix fashion to form an imaging array. FIG. 1B shows a schematic equivalent circuit of a 3×3 pixels imaging array that can be used for a general radiation detection device. The data line 140 is shared between pixels in each column and is connected to the readout electronics. The scan line 130 is shared between pixels in each row and is connected to the driving electronics. The driving circuitry provides the appropriate signals on scan lines 130 to release the signals stored on the floating nodes 160 of pixels 100 to the data lines 140 one row at a time, usually in sequence.
Two sensor pixel 100 architectures well known in the imaging art are the coplanar pixel 200 and the vertically-integrated pixel 210 shown in FIGS. 2A, 2C and 2B, 2D, respectively. Coplanar pixel 200 differs from the vertically-integrated pixel 210 in that a portion of the photoelectric conversion device 120 is not situated on top of the switching element 110. FIG. 2A and FIG. 2C illustrate a top-down view and a cross-sectional view of a coplanar pixel 200, respectively. FIG. 2B and FIG. 2D illustrates the top-down view and cross-sectional view of a vertically-integrated pixel 210, respectively. In both cases, the switching element 110 is an inverted-staggered back-channel-etch (BCE) TFT commonly found in the liquid crystal display (LCD) backplane technology, and the photoelectric conversion device 120 is a p-i-n photodiode. The approximate regions of the switching elements 110 and the photoelectric conversion elements 120 are highlighted by the thick bounding boxes in FIGS. 2A-2D. In the coplanar pixel design 200, the TFT 110 and photodiode 120 are situated parallel to one another on a substrate 220, for example, glass. In the vertically-integrated pixel design 210, a relatively thick layer of inter-layer dielectric insulator material 230 is sandwiched between a portion of the photodiode 120 and the TFT 110.
There is a general desire to achieve higher pixel sensitivity; doing so could either lower the required radiation dosage while maintaining similar readout signal levels, or maintaining the same radiation dosage to obtain higher readout signal levels. The output signal-to-noise ratio (SNR) is boosted with an increase in output signal level while having the same output noise level. Higher output SNR can provide improved discrimination between features of interest and unwanted noise in the image.
One key factor that influences the pixel sensitivity is the pixel fill factor (FF). FIG. 3A shows a top-down view of a coplanar pixel 200 and FIG. 3B shows a top-down view of a vertically-integrated pixel 210. The fill factor of a pixel can be approximated as the ratio of the photosensitive area of the pixel 300 to the total pixel area (marked by the pixel boundary in FIG. 3A and FIG. 3B). In both FIG. 3A and FIG. 3B, the photosensitive area 300 of the pixel is highlighted by thick bounding boxes. Portions of the photoelectric conversion element 120 covered by the bias line 150 (marked by hatched region 310) are often not considered as photosensitive since the composition of metal used for the bias line 150 is usually substantially opaque to incident photons energies. As shown by comparing FIG. 3A and FIG. 3B, generally, higher fill factors can be obtained in vertically-integrated pixel 210 as compared to coplanar pixel 200. However, issues related with the vertically-integrated pixel architecture 210, for example, added layer stress and degraded sensor performance due to large variation in underlying topology (illustrated in FIG. 2D), can make the coplanar pixel 200 the preferred pixel architecture. Therefore, there is a strong desire to achieve higher pixel fill factor for coplanar pixels.
Due to various limitations imposed by the fabrication process of the imaging array, for example, minimum feature size, the pixel fill factor does not stay constant with varying pixel sizes. The pixel fill factor generally decreases with decreasing pixel pitch and this reduction can be more severe for smaller pixel sizes. Imaging array resolution requirements for single-shot general radiography applications for example, can require pixel pitch to be in the range of about 120 μm to about 150 μm; while for specialized applications such as mammography, the demand for finer pixel pitch can be in the range of about 40 μm to about 80 μm.
In view of the issues described above, the object of the present invention is to improve the coplanar pixel sensitivity by achieving higher fill factor. Another object of the present invention is to improve the coplanar pixel sensitivity for smaller pixel sizes by achieving smaller reduction in pixel fill factor with reduction in pixel size.